This invention pertains to staged combustion furnaces used for burning pyrolysis gas and char to produce hot combustion gases and generate power from coal in a process utilizing both air and steam turbines. It pertains particularly to two-stage furnaces in which a pyrolysis gas derived from coal is combusted in a first stage containing a high temperature compressed air heat exchanger and a steam superheater. Char from the first stage and coal as required, are combusted in a second stage furnace in which the compressed air is preheated and steam is initially superheated.
In a high-efficiency coal-fired combined power cycle using a high temperature (Brayton Cycle) air turbine together with a steam (Rankine Cycle) turbine for generating electric power on both turbine shafts, it is desirable to raise the temperature of the Brayton Cycle working fluid (air) to at least 1800.degree. F. This requires high-temperature combustion gases which, when derived directly from coal, will be at a temperature at which both vaporized and molten ash constituents are present. Such ash constituents in the hot gases could cause corrosion and plugging of heat exchanger tubes, which need to be made of a suitable high temperature material e.g. a ceramic such as silicon carbide for the air heat exchanger, and metallic alloy for a steam superheater. Because the cost of such heat exchanger tubing is high, a closely packed heat exchanger is desirable so as to minimize the amount of heat transfer surface required.
Processes utilizing staged combustion of coal are known, as disclosed by U.S. Pat. No. 3,840,354 to Donath, U.S. Pat. No. 4,229,185 and 4,322,222 to Sass, U.S. Pat. No. 4,312,639 to Johnson, U.S. Pat. No. 4,469,488 to Calderon, and U.S. Pat. No. 4,900,429 to Richardson. However, these processes and associated furnace equipment have various deficiencies, so that improved more compact furnace combustion equipment and a process for more efficient staged combustion of coal to generate power have been needed. An improved combustion furnace assembly including a pyrolyzer unit and two-staged combustion furnaces has now been developed, in which coal is first pyrolyzed to produce pyrolysis gas and char. The pyrolysis gas is used to fire a first stage furnace containing an air heater and high temperature steam superheater. The char is used to fuel a second stage furnace in which the combustion gas therein is advantageously quenched by the primary combustion gases from the first stage furnace.